[Q-e-developers] Problems converging anions in vacuum

[Nicola Marzari]

Thanks to all! We'll keep looking into this.

We are aware of DFT issues on self-interaction, loosely
bound anions, and periodic-boundary interactions (these are
actually research areas in the group).

I think the problem here is different, although I'll double
check with Oliviero that we are not on the verge of losing an
electron to vacuum (I'm pretty sure this was checked).

Namely, the calculations for all these anions are almost perfectly
converged, and then in one iteration c_bands gives "1 eigenvalues not 
converged", and after that the calculation diverges/crashes (see below).

If one looks at the charge density at the first iteration with c_bands
not converged, one can see little ripples at high frequency appearing
in the total charge density. Also the negative rho shoots up by at least 
one/two orders of magnitude. I presume these must be driven by the
fact that one of the eigenstates in produced by c_bands is now
meaningless - so the issue is why c_bands with CG close to convergence
can fail - that would be my question.

Oliviero and Davide noted that a different O pseudopotential
does not give rise to this error, so I presume it could be due
to a ghost state almost overlapping with the least bound electron,
but it is surprising for this to happen in a large number of
anions so close to perfect convergence.

			nic



      iteration # 27     ecut=    30.00 Ry     beta=0.40
      CG style diagonalization
      c_bands:  1 eigenvalues not converged
      ethr =  3.22E-10,  avg # of iterations =  6.0

      negative rho (up, down):  0.462E-02 0.000E+00

      add environment contribution to local potential

      total cpu time spent up to now is     2842.4 secs

      total energy              =     -94.77428965 Ry
      Harris-Foulkes estimate   =     -94.77428964 Ry
      estimated scf accuracy    <       0.00000028 Ry

      iteration # 28     ecut=    30.00 Ry     beta=0.40
      CG style diagonalization
      c_bands:  1 eigenvalues not converged
      ethr =  3.22E-10,  avg # of iterations =  5.3

      negative rho (up, down):  0.116E+00 0.000E+00

      total cpu time spent up to now is     2921.7 secs

      total energy              =     -94.78295397 Ry
      Harris-Foulkes estimate   =     -94.77524064 Ry
      estimated scf accuracy    <       1.78307693 Ry


> --
> Goranka Bilalbegovic
> Department of Physics, Faculty of Science,
> University of Zagreb, Croatia
>
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-- 

----------------------------------------------------------------------
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL